Plastics materials are increasingly being used in packaging applications, replacing glass and metal. However, plastic materials generally have inferior vapor barrier properties compared to glass and metal containers. Accordingly, plastic packages generally admit oxygen into the package at a faster rate, show higher aroma loss, higher water vapor loss, and greater CO2 loss in case of carbonated beverage packages. Other advantages afforded by plastics such as lighter weight, lower cost, the ability to form attractive container shapes continues to sustain or increase the share of plastics in packaging applications. Improved test methods are therefore constantly being developed to measure plastics barrier properties.
When the permeant of interest is CO2, for example, in measuring shelf life of carbonated beverage packages, two broad categories of methods are presently used. The first methods measure retained CO2 in the packages by periodically measuring CO2 in a simulated shelf-life test. The second methods measure the rate of CO2 loss from the package at a point in time and at a measured driving force (partial pressure) in the container. For typical packages of interest, methods measuring retained CO2 have several deficiencies. The CO2 content changes only very slightly (10 to 15%) during the shelf life, demanding exquisite sensitivity in the detection mechanism because one is required to measure such small changes over a large offset. In addition, such small signal changes occur over very long periods of time (8 to 12 weeks—essentially the expected shelf life of the package) rendering these methods useless for rapid permeation assessment. As such, these methods are of little predictive value. The CO2 loss rate measurement methods, which in principle can measure the signal without having to measure against a large offset as in the retained CO2 methods, in their current implementation have several disadvantages as discussed below. Although loss rate methods are faster than the retained CO2 methods, they are often inaccurate and shelf life predictions from such methods often do not concur with measurements using the retained CO2 tests. The present invention overcomes deficiencies of the loss rate measurement methods in prior art and thus provides a method that is not only very rapid, but also predicts shelf life with accuracy.
Several techniques and apparatus for measuring oxygen permeability of plastic films are patented and published. Usually, the film sample of interest is sandwiched between two test chambers using appropriate sealing mechanisms. One chamber is exposed to the permeant gas of interest in high concentrations while the second chamber is purged continuously with an inert carrier gas. Any permeated permeant from the first chamber is thus transported to a permeant detector located outside these permeation chambers. The detector itself is usually a sensor that is capable of measuring extremely small concentrations of the permeant. While in principle the method can also be applied to testing containers, such a configuration does not lend itself to nondestructive testing of whole containers.
Use of such a continuous purging system is also disadvantageous for other reasons. The concentration of permeated permeant is often extremely small in the second chamber, especially when the sample material is a good barrier. Measuring such low concentrations of the permeants reliably is a challenge for most sensors. Furthermore, such low permeant concentrations are diluted several fold when a carrier gas is used to transport the permeated permeant to the remote detector. The objective of the measurement, the true permeation rate is directly related to the product of carrier gas flow rate and the detector response. For accurate estimation of permeation rates, it becomes essential that the flow rate of the carrier gas be accurately and robustly controlled, and also that the detector be capable of exquisite sensitivity.
In addition, the permeability of many barrier materials is highly sensitive to temperature and humidity conditions at the time of the test. For example, CO2 permeability of EVAL-F, a commercially available barrier material, increases about 30-fold when the relative humidity increases from 50% to 90%. Most beverage packages contain 100% relative humidity inside the package. It is of interest to measure permeability under precisely defined high external humidity conditions. To my knowledge, the relevant prior art does not teach how testing at precisely controlled high relative humidities is to be conducted. Precise control of humidity, especially at high humidity levels is difficult to achieve in current systems mainly because of the difficulty associated with controlling humidity in a continuous purge stream. Further, humidity control in the first chamber, where the permeant source is provided is even more difficult, and is left uncontrolled.
An example of non-flowing systems used for package permeability testing is described in U.S. patent application U.S. 20020194899A1 published Dec. 26, 2002, titled “Method and Device for the Determination of the Gas Permeability of a Container” (equivalent world patent WO 0148452). Here, packages containing high partial pressures of the permeant are placed in a confinement chamber in such a way as to minimize the spacing between the package and the chamber. As the permeant permeates to this inter-space, the pressure therein increases and is measured with a sensitive pressure gage. The disadvantage with such a system is that plastic containers expand under pressure. Thus, the volume of the inter-space can change significantly causing erroneous pressure readings. Thus, while this method could be faster than the retained CO2 methods, its practical utility is limited to measuring relative barrier property differences between different packages rather than absolute permeability measurements for each package independently that is necessary for shelf life predictions. Further, since the method relies heavily on minimizing the volume of the inter-space between the given confining chamber and the package, significant deviations in the test package shape or size will require completely different chambers to be used. Lastly, methods for accurate humidity control that are critical for most barrier property testing are not taught in the above described U.S. patent application.
Another example of a non-flowing system used in the field of vacuum insulation panel testing is described in U.S. Pat. No. 5,345,814, issued 13 Sep. 1994, and entitled “Method and Apparatus for Testing Vacuum Insulation Panel Quality”. Vacuum insulation panels are prepared by evacuating air and introducing small quantities of helium tracer gas into the panels just prior to sealing. The panels are placed in an evacuated chamber at an even lower pressure than within the panels and the helium is allowed to flow out of the insulation panel into the chamber space. The integrity of the panels is assessed by sampling the chamber gases to measure the rate of helium accumulation. While this approach overcomes some of the limitations of the continuous purge systems, it uses a flow or a sniffer type detector, and relies on the contents of the chamber to be exhausted with an elaborate equipment set up. Most critically, the '814 patent does not teach ways of temperature and humidity control in package testing.
Further, no prior art references teach a method for measuring sorption of the permeant by the packaging materials empirically.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide new and improved apparatus and techniques for measuring package permeability.
Another object of the invention is to provide new and improved apparatus and techniques for measuring package permeant sorption.
Another object of the invention is to provide new and improved package permeability measuring apparatus and techniques that are extremely accurate.
Another object of the invention is to provide new and improved package permeability measuring apparatus and techniques that are substantially faster than prior art apparatus and techniques.
A further object of the invention is to provide new and improved package permeability measuring apparatus and techniques that can be used on substantially any types, shapes, and sizes of packages.
A further object of the invention is to provide new and improved apparatus and techniques for measuring package permeant sorption wherein accurate control of humidity and temperature are better achieved.